1. Field of the Invention
This invention relates to strobe controlling apparatus and more particularly to a strobe controlling apparatus for properly controlling the exposure to prevent the over exposure at the time of strobe flash photographing an object at a short distance and to deepen the photographed field depth on the short distance side.
2. Related Background Art
As is well known, in the exposure control at the time of strobe flash photographing with a lens shutter camera such as a compact camera, there is generally used a so-called flashmatic system of automatic exposure controlling system which is of a guide number (abbreviated as GNo hereinafter) of a fixed value (substantially full flash GNo)(See U.S. Pat. No. 4,899,191). In this flashmatic system (abbreviated as an FM system hereinafter), on the basis of the fact that the guide number, that is, the product of the diaphragm and irradiated distance is constant, an object distance (mentioned as D [m] hereinafter) by range finding information measured by a range finding means within the camera is used for the strobe guide number GNo of a fixed value to control the diaphragm value (mentioned as FNo hereinafter) of the lens and to obtain a proper exposure. That is to say, the FNo is determined from the formula ##EQU1## to control the diaphragm. Here, S represents an ISO sensitivity of the film and S(100) represents a film sensitivity at the time of ISO 100. When the above-mentioned formula (1) is modified, it will be ##EQU2##
The FMv diagram for determining a proper exposure when this FM system is used shall be explained in the following with FIG. 15. As a method of determining a proper exposure by an external light (light other than the strobe light) is generally used the formula of an apex system shown below: EQU EV=AB+TV=BV+SV
wherein EV represents an exposure value, AV represents an aperture value and is shown by the following formula: EQU AV=log.sub.2 (FNo).sup.2 ( 3)
TV represents a time value and is shown by the following formula: EQU TV=log.sub.2 (1/T) (4)
wherein T represents a shutter second time, that is, an effective exposure time (in seconds) of the shutter, BV represents a brightness value (or a luminance value) and is shown by the following formula: EQU BV=log.sub.2 {B / (K.multidot.N)}
wherein B represents a luminance of the object to be photographed, K and N are exposure constants determined by the camera and the K value is usually 1.3 in this camera and further SV represents an ASA (ISO) speed value and a sensitivity of the film and is shown by the following formula: EQU SV=log.sub.2 (N.multidot.S) (5)
wherein S represents an ISO sensitivity of the film and, for example, in case S is 100 and N is 0.32, the SV value will be 5.
If the specification of a camera is represented by the EV diagram shown in the upper part of FIG. 15, the relation between the above-mentioned AV value and TV value will be determined by the thick line L in the middle of the above-mentioned FIG. 15 and the exposure condition in case no strobe is used will be determined on the basis of the EV value corresponding to the SV value and BV value. That is to say, if BV=5 and SV=5, EV will be: EQU EV=5+5=10
and, from the intersection of the equal EV line of EV=10 and the thick line L, the exposure condition will be controlled with AV=4.5 and TV=5.5.
The case of using a strobe shall be explained in the following. In this camera, the flashmatic flash point will be operatively connected between 1/100 second and 1/500 second when the effective exposure time shows a .gamma. conversion point. When this is converted to an AV value, it will be: EQU 4.5.ltoreq.AV.ltoreq.6.7
When both sides of the above-mentioned formula (1) are squared and their logarithms are taken and arranged, EQU log.sub.2 GNo.sup.2 =log.sub.2 FNo.sup.2 +log.sub.2 D.sup.2 +log.sub.2 S (100)-log.sub.2 S (6)
will be obtained. When the GV value of the guide number information and DV value of the distance information are set to be: EQU GV=log.sub.2 GNo.sup.2 and EQU DV=log.sub.2 D.sup.2
and further when the formulae (3) and (5) are substituted in the formula (6), EQU GV=AV+DV-SV+5 (7)
will be made. In this camera, as the GNo is only of a full flash (14 as GNo), the GV value of this full flash shall be GVm (=7.6). Also, as films of ISO 400 tend to be usually mostly used, ISO 400, that is, SV=7 is substituted in the above-mentioned formula (7) to obtain EQU GVm=AV+DV-7+5
Therefore, ##EQU3##
The operatively connecting range of the AV value will become: EQU 4.5.ltoreq.AV.ltoreq.6.7
When the formula (8) is substituted in the above formula, the operatively connecting range of the DV value will be EQU 3.1.ltoreq.DV.ltoreq.5.1
Therefore, in the FMv diagram (GNo=14 constant) in FIG. 15, the exposure will be proper in the region Rg2 but will be over-exposed in the region Rg1 (of about 3 m to a very short distance) and will be under-exposed in the region Rg3 (of about 6 m to a long distance).
Thus, in the exposure controlling system in the conventional lens shutter camera, there has been a defect that the exposure is likely to be over-exposed at a very short distance. As the frequency of using high sensitivity films tends to increase with the improvement of the quality of the high sensitivity films, the above-mentioned defect can be said to be a great problem.
In this case, there is an over exposure preventing means by directly measuring the strobe light and stopping the flash when a required light amount is reached. However, by this means, the flash timing can not be determined. That is to say, the flash amount at the time of a fixed diaphragm is only controlled. Therefore, the operatively connecting range will be limited only to the range in which the direct light measurement follows.
In this kind of conventional technical means, in the case of controlling the exposure in the FM system by using a high ISO sensitivity film in such camera adopting a lens shutter as a compact camera, the exposure will be proper in the region Rg2 in the above-mentioned FIG. 15 but will be over-exposed in the region Rg1 on the short distance side. Therefore, by the means of directly ensuring the strobe light and stopping the flash when a required light amount is reached, only the flash amount at the time of a fixed diaphragm value has been able to be controlled.